Multiconductor flat cables are widely used in the electronics industry. Typically, such cables comprise a polyimide base layer and flat thin leads disposed thereon, with a top cover laminated to the base layer. The leads are most often formed by subtractive processes wherein a copper lamination on the base layer is etched to form a plurality of parallel conductors for the full length of the base layer. Other processes (i.e., additive) are sometimes used to provide the parallel conductors.
One process often used to create the connection between the cable leads and the bond pads of the device is a thermal compression bond, which uses heat and pressure to join the cable leads and the device bond pads in gang form.
Historically, when the cable lead density was low and the lead sizes were large, the leads extended beyond the end of the base and cover layers. These leads were large enough so that their mechanical strength was sufficient to tolerate most normal production handling. Also, the leads were widely separated and the pads to which the leads were bonded were oversized to accommodate small misalignment of the cable leads so as to still be within registration on the bond pads.
More recently, the requirements have increased for more leads in the cable without increasing the size of the cable, requiring increased cable lead density. Thus, the lead size has been reduced, and required a complimentary reduction in bond pad size, and therefore required improved registration of the cable with respect to the bond pads. To meet this requirement for improved registration with a smaller and mechanically weaker lead, a new cable was developed. The cable polyimide base and cover layers were extended to the end of the leads to provide improved mechanical strength. Then, a laser system was used to ablate an area of the cable and open a window through both the polyimide base and cover layers near the en d o f th e cable to fully expose the cable leads. The remaining polyimide "frame" around the leads provided the additional mechanical strength and the window provided full access to the leads. The thermal compression bond anvil then used this window in the cable to contact the leads on one side and bond them to the bond pads on the other side.
Today, there is a demand for further increases in cable densities with even smaller leads and smaller bond pads. Attempts to extend the laser window technology to increased densities is resulting in increased handling damage and part rejection, driving up costs. Conductors are becoming only 2 mils in width and 0.7 mils thick and therefore are increasingly fragile. Thus, the lack of mechanical support is the major issue, causing the increased handling damage.
Another problem surfacing with the smaller conductors and corresponding bond pads is that residue from the laser ablation may condense back onto the conductors, requiring an aggressive cleaning process that attacks other areas of the cable and preventing a perfect bond and resulting in part rejection.
Thus, what is needed is a way of providing mechanical support for the fragile conductors of a multiconductor flat cable without interfering with the bonding process.